LeetCode 面试题 16.02. 单词频率

设计一个方法，找出任意指定单词在一本书中的出现频率。
你的实现应该支持如下操作：
WordsFrequency(book)构造函数，参数为字符串数组构成的一本书
get(word)查询指定单词在书中出现的频率
示例：
WordsFrequency wordsFrequency = new WordsFrequency({"i", "have", "an", "apple", "he", "have", "a", "pen"});
wordsFrequency.get("you"); //返回0，"you"没有出现过
wordsFrequency.get("have"); //返回2，"have"出现2次
wordsFrequency.get("an"); //返回1
wordsFrequency.get("apple"); //返回1
wordsFrequency.get("pen"); //返回1
提示：
book[i]中只包含小写字母
1 <= book.length <= 100000
1 <= book[i].length <= 10
get函数的调用次数不会超过100000

class WordsFrequency:
    def __init__(self, book: List[str]):
        self.retDict = defaultdict(int)
        for i in book:
            self.retDict[i] += 1
    def get(self, word: str) -> int:
        return self.retDict[word]
# Your WordsFrequency object will be instantiated and called as such:
# obj = WordsFrequency(book)
# param_1 = obj.get(word)